1. Field of the Invention
The present invention relates to wireless telecommunications. More particularly, the present invention relates to a new cellular architecture for cellular systems such as Code Division Multiple Access (CDMA) Smart Antenna Array Systems.
2. Description of the Related Art
In wireless communications systems, including second generation (2G) and third generation (3G) systems, a viable option for improving system capacity is to employ a smart antenna array in both the forward link (from the base station to the mobile) and the reverse link (from the mobile to the base station). As used herein, the term smart antenna refers to an antenna that can detect the angle at which a signal is received, and that can transmit a focused beam in a desired direction or directions as well as selectively receive a transmitted signal from a desired direction or directions. In the reverse link, the base station can achieve improved capacity by suppressing the interfering signals from different direction of arrival angles (DOAs) by using spatial diversity. In the forward link, the base station can use the DOA capabilities of the reverse link system to focus a directional beam to a user at any given angle and suppress the signal to other users.
One side effect of smart antenna arrays is the non-uniform gain pattern that results in a multi-user interference environment. When a cluster of cells is arranged in the conventional Wide Beam Tri-sector Cell (xe2x80x9cWBTCxe2x80x9d) architecture, the result is a large soft handoff region which can degrade the system leading to reduced capacity.
What is needed is a new architecture for cellular systems employing smart antennas in which the soft handoff area is reduced relative to the conventional WBTC architecture.
The new cellular architecture is called the Interleaved Wide Beam Tri-sector Cell (IWBTC) architecture in which base stations have an orientation and are arranged in a regular pattern based on a three sector cell with each sector in the shape of a hexagon. In this architecture, neighboring base stations are aligned in a first direction and staggered, preferably by approximately 10.89 degrees, in a second direction perpendicular to the first direction. In addition, neighboring base stations in the first direction have the same orientation, while neighboring base stations in the second direction are rotated by approximately sixty degrees with respect to each other. The architecture of the invention may be used in any cellular system, but is particularly well suited to CDMA systems which do not use frequency division (that is, in CDMA systems in which neighboring base stations transmit over the same range of frequencies), and which employ smart antennas. The architecture takes advantage of the smart antenna array characteristics (when CDMA base stations employ the smart antenna array in the forward and reverse channels), resulting in a significantly decreased soft handoff area and thus a significant improvement as compared to the conventional WBTC architecture.
In another aspect of the invention, a system and method for evaluating cellular architecture performance are provided. In the method, equations for defining the smart antenna array are provided, then the interference is calculated, and finally an equation for asymptotic capacity is provided to compare the performance of the IWBTC and WBTC architectures.
In still another aspect of the invention, the smart antenna is used to direct a transmission to a mobile user in three different directions: a first direction on which the signal strength received from the mobile unit is highest (the xe2x80x9cprimary multipathxe2x80x9d), as well as the directions on either side of the first direction on which the next-highest signal strength is received (the xe2x80x9csecondary multipathsxe2x80x9d). In preferred embodiments, one half of the transmitted power is directed toward the primary multipath, while one quarter of the transmitted power is directed toward each of the two secondary multipaths.